Characterization of morphology and active agent mobility within hybrid silica sol-gel composites

The porous properties of sol-gels make them attractive choices for developing functional materials requiring the controlled release of entrapped active agents for a range of applications that include protective coatings, sensors, self-healing films, and drug delivery. In this study, we investigate the properties of sodium benzoate as a model active agent entrapped within a mesoporous silica matrix formed from the biocompatible precursor diglycerylsilane (DGS). Quasi-elastic neutron scattering (QENS) is used to measure the mobility and diffusion of the mobile species in the gels, whereas small-angle neutron scattering (SANS) provides key information on the structural features of the gels. QENS measurements indicate that the diffusivity of the benzoate species decreases approximately 4-fold, from 14.3 × 10 ^-10 m ² s ^-1 in bulk solution to 3.3 × 10 ^-10 m ² s ^-1 following entrapment within DGS-derived sol-gels. A similar trend is exhibited by water entrapped within the porous network. However, subsequent detailed analysis reveals that the diffusivity of water is not solely influenced by simple confinement in the gels per se but rather by interaction(s) with benzoate species. It can be concluded that confinement in the mesoporous silica gels produces a pronounced suppression in the diffusivity of benzoate but, contrary to conventional wisdom, it appears that it is the composition of the mobile phase that most affects the diffusivity of D ₂O both within the mesoporous silica gels and in the bulk. Structural analysis of the gels using SANS confirms that the gels comprise highly branched structures with relatively large pore sizes in the mesoporous regime. There is no evidence for the formation of benzoate aggregates greater than 2 nm in size within the gels despite the fact that dynamics measurements reveal a decreased diffusivity for benzoate, which might reasonably be expected to occur as a result of aggregation and/or an increased interaction with the sol-gel matrix. The ability to characterize the distribution and diffusion of guests mobilized in porous host carriers remains a major scientific bottleneck in controlled-release research. In this fashion, this investigation nicely highlights the fact that neutron scattering techniques represent a set of powerful, complementary, and underutilized tools to characterize and unlock the complex structural and dynamic features of complex materials of interest in controlled release and elsewhere.